VARIATION IN RUBBER-CONTENT OF ASCLEPIAS AND APOCYNUM. 49 



on, but it may now be said that probably both heredity and environ- 

 ment are concerned; that is, that each species is composed of many 

 strains, each with a different rubber-producing capacity, and that at 

 the same time the amount of rubber present in plants of each strain 

 is modified by the local environment. If this assumption is correct, 

 the chances of improvement through selection and breeding, and also 

 through the manipulation of the environment, are greatly enhanced. 

 There is a wide individual variation in plants growing under uniform 

 conditions, as is indicated, although not finally proven, by detailed 

 studies made on several species. For example, 4 plants of Asclepias 

 sullivanti growing near Lincoln, Nebraska, in what appeared to be 

 uniform soil and only a few feet from one another, were all gathered 

 at the same time (August 28, 1920) and analyzed separately, with the 

 following results: 



TABLE 13.- 



-Variation in rubber-content of plants of Asclepias sullivanti 

 growing under apparently uniform conditions. 



The extreme variation from 1.1 to 5.4 per cent in the leaves, as indi- 

 cated in this table, can scarcely be attributed to what slight difference 

 there might have been between the environments of the two plants. 

 Similar evidence is deduced from the analyses of 3 plants of Asclepias 

 subulata gathered at Sentinel, Arizona. The rubber-content of the 

 stems of these plants, the foliage being so sparse as to be negligible, 

 was found to be 2.6, 3.5, and 4.4 per cent respectively. In this case 

 the differences are conceivably due to slight inequalities in the soil 

 surface and hence in the water relation. 



A third series of samples was taken to test this point, this time of 

 Asclepias mexicana. Three plants (910, 911, and 912) were selected 

 near Mount Diablo, California, all apparently alike and all from 

 ivithin a foot from one another in a field where the herbaceous vegeta- 

 tion exhibited great uniformity in its growth. The leaves of these 

 plants were analyzed and found to contain in one case 2.8 per cent, 



